Process to Produce a Gas Oil by Catlaytic Cracking of a Fisher-Tropsch Product

ABSTRACT

Process to prepare a gas oil, by (a) isolating from a Fischer-Tropsch synthesis product a first gas oil fraction and a fraction boiling above the gas oil fraction, (b) contacting the heavier fraction with a catalyst system comprising a catalyst, which catalyst comprises an acidic matrix and a large pore molecular sieve in a riser reactor at a temperature of between 450 and 650° C. at a contact time of between 1 and 10 seconds and at a catalyst to oil ratio of between 2 and 20 kg/kg, (c) isolating from the product of step (b) a second gas oil fraction; (d) combining the first gas oil fraction with the second gas oil.

FIELD OF THE INVENTION

The invention relates to a process to prepare a gas oil, in combinationwith a gasoline, by catalytic cracking of a Fischer-Tropsch product.

BACKGROUND OF THE INVENTION

It is known that paraffinic products boiling in the gas oil range can beprepared from a Fischer-Tropsch derived synthesis product. However,Preparing a gasoline having an acceptable octane number, and aparaffinic gas oil, from a Fischer-Tropsch product, using a singleconversion process, is not straightforward. This because theFischer-Tropsch product as such consists for a large portion of normalparaffins which have a low octane value or contribution. Variouspublications are known which describe catalytic cracking as a process toprepare a gasoline having an acceptable octane value from aFischer-Tropsch product. For example U.S. Pat. No. 4,684,756 discloses aprocess to prepare a gasoline fraction directly by catalytic cracking ofa Fischer-Tropsch wax as obtained in an iron catalysed Fischer-Tropschprocess. The gasoline yield is 57.2 wt %.

A disadvantage of some of the above processes involving catalyticcracking is that the cetane number of the gas oil fraction, which isproduced in combination with the gasoline, is too low, and the gas oilyield is low.

The object of the present invention is to prepare a high qualityparaffinic gas oil in a catalytic cracking process of a Fischer-Tropschproduct which process has as the main product a gasoline.

SUMMARY OF THE INVENTION

Process to prepare a gas oil, by

-   (a) isolating from a Fischer-Tropsch synthesis product a first gas    oil fraction and a fraction boiling above the gas oil fraction,-   (b) contacting the heavier fraction with a catalyst system    comprising a catalyst, which catalyst comprises an acidic matrix and    a large pore molecular sieve in a riser reactor at a temperature of    between 450 and 650° C. at a contact time of between 1 and 10    seconds and at a catalyst to oil ratio of between 2 and 20 kg/kg,-   (c) isolating from the product of step (b) a second gas oil    fraction;-   (d) combining the first gas oil fraction with the second gas oil.

DETAILED DESCRIPTION OF THE INVENTION

Applicants found that the first gas oil fraction, obtained in step (a),will improve the cetane number of the second gas oil obtained bycatalytically cracking a Fischer-Tropsch synthesis product. In apreferred embodiment, a relatively heavy Fischer-Tropsch product is usedas feed to the catalytic cracking step (b). The enrichment of thecatalytically cracked gas oil fraction with paraffins, as obtained instep (a), increases the cetane number to the level that makes the gasoil suitable as a diesel fuel blend component. Another advantage is thatuse can be made of well-known processes known for fluid catalyticcracking (FCC), step (b).

The Fischer-Tropsch synthesis product may in principle be any reactionproduct as obtained when performing the well know Fischer-Tropschsynthesis reaction. Preferably use is made of a relatively heavyFischer-Tropsch product in step (b). This heavy feed preferably has atleast 30 wt %, preferably at least 50 wt %, and more preferably at least55 wt % of compounds having at least 30 carbon atoms. Furthermore theweight ratio of compounds having at least 60 or more carbon atoms andcompounds having at least 30 carbon atoms of the Fischer-Tropsch productis at least 0.2, preferably at least 0.4 and more preferably at least0.55. Preferably the Fischer-Tropsch product comprises a C₂₀+ fractionhaving an ASF-alpha value (Anderson-Schulz-Flory chain growth factor) ofat least 0.925, preferably at least 0.935, more preferably at least0.945, even more preferably at least 0.955.

The initial boiling point of the Fischer-Tropsch product used in step(b) may suitably range from below 200 up to 450° C. Preferably theinitial boiling point is between 300 and 450° C. in case all compoundshaving a boiling point in the gas oil range are separated from theFischer-Tropsch synthesis product before the Fischer-Tropsch synthesisproduct is used in step (b). Applicants found that a high yield to gasoil can be achieved starting from such a Fischer-Tropsch product, thusexcluding the Fischer-Tropsch fractions boiling in the gas oil range.The relatively heavy Fischer-Tropsch synthesis product can be obtainedby any process, which yields a relatively heavy Fischer-Tropsch product.Not all Fischer-Tropsch processes yield such a heavy product. Preferredprocesses are the cobalt catalysed Fischer-Tropsch processes. An exampleof a suitable Fischer-Tropsch process is described in WO-A-9934917 andin AU-A-698391. These processes may yield a Fischer-Tropsch product asdescribed above.

A preferred catalyst to be used to obtain the relatively heavyFischer-Tropsch product is suitably a cobalt-containing catalyst asobtainable by (aa) mixing (1) titania or a titania precursor, (2) aliquid, and (3) a cobalt compound, which is at least partially insolublein the amount of liquid used, to form a mixture; (bb) shaping and dryingof the mixture thus obtained; and (cc) calcination of the compositionthus obtained.

Preferably at least 50 weight percent of the cobalt compound isinsoluble in the amount of liquid used, more preferably at least 70weight percent, and even more preferably at least 80 weight percent, andmost preferably at least 90 weight percent. Preferably the cobaltcompound is metallic cobalt powder, cobalt hydroxide or an cobalt oxide,more preferably Co(OH)₂ or Co₃O₄. Preferably the cobalt compound is usedin an amount of up to 60 weight percent of the amount of refractoryoxide, more preferably between 10 and 40 wt percent. Preferably thecatalyst comprises at least one promoter metal, preferably manganese,vanadium, rhenium, ruthenium, zirconium, titanium or chromium, mostpreferably manganese. The promoter metal(s) is preferably used in suchan amount that the atomic ratio of cobalt and promoter metal is at least4, more preferably at least 5. Suitably at least one promoter metalcompound is present in step (aa). Suitably the cobalt compound isobtained by precipitation, optionally followed by calcination.Preferably the cobalt compound and at least one of the compounds ofpromoter metal are obtained by co-precipitation, more preferably byco-precipitation at constant pH. Preferably the cobalt compound isprecipitated in the presence of at least a part of the titania or thetitania precursor, preferably in the presence of all titania or titaniaprecursor. Preferably the mixing in step (aa) is performed by kneadingor mulling. The thus obtained mixture is subsequently shaped bypelletising, extrusion, granulating or crushing, preferably byextrusion. Preferably the mixture obtained has a solids content in therange of from 30 to 90% by weight, preferably of from 50 to 80% byweight. Preferably the mixture formed in step (aa) is a slurry and theslurry thus-obtained is shaped and dried by spray-drying. Preferably theslurry obtained has a solids content in the range of from 1 to 30% byweight, more preferably of from 5 to 20% by weight. Preferably thecalcination is carried out at a temperature between 400 and 750° C.,more preferably between 500 and 650° C. Further details are described inWO-A-9934917.

The Fischer-Tropsch process is typically carried out at a temperature inthe range from 125 to 350° C., preferably 175 to 275° C. The pressure istypically in the range from 5 to 150 bar abs., preferably from 5 to 80bar abs., in particular from 5 to 70 bar abs. Hydrogen (H₂) and carbonmonoxide (synthesis gas) is typically fed to the process at a molarratio in the range from 0.5 to 2.5. The gas hourly space velocity (GHSV)of the synthesis gas in the process of the present invention may varywithin wide ranges and is typically in the range from 400 to 10000Nl/l/h, for example from 400 to 4000 Nl/l/h. The term GHSV is well knownin the art, and relates to the volume of synthesis gas in Nl, i.e.litres at STP conditions (0° C. and 1 bar abs), which is contacted inone hour with one litre of catalyst particles, i.e. excludinginterparticular void spaces. In the case of a fixed catalyst bed, theGHSV may also be expressed as per litre of catalyst bed, i.e. includinginterparticular void space. The Fischer-Tropsch synthesis can beperformed in a slurry reactor or preferably in a fixed bed. Furtherdetails are described in WO-A-9934917.

Synthesis gas may be obtained by well known processes like partialoxidation and steam reforming and combinations of these processesstarting with a (hydro) carbon feedstock. Examples of possiblefeedstocks are natural gas, associated gas, refinery off-gas, residualfractions of crude oil, coal, pet coke and biomass, for example wood.Partial oxidation may be catalysed or non-catalyzed. Steam reforming maybe for example conventional steam reforming, autothermal (ATR) reformingand convective steam reforming. Examples of suitable partial oxidationprocesses are the Shell Gasification Process and the Shell CoalGasification Process.

The Fischer-Tropsch product will contain no or very little sulphur andnitrogen containing compounds. This is typical for a product derivedfrom a Fischer-Tropsch reaction, which uses synthesis gas containingalmost no impurities. Sulphur and nitrogen levels will generally bebelow the detection limits, which are currently 5 ppm for sulphur and 1ppm for nitrogen. The Fischer-Tropsch product can advantageously bedirectly used in step (a) without having to hydrotreat the feed toremove olefins and/or oxygenates.

The catalyst system used in step (b) will at least comprise of acatalyst comprising of a matrix and a large pore molecular sieve.Examples of suitable large pore molecular sieves are of the faujasite(FAU) type as for example Zeolite Y, Ultra Stable Zeolite Y and ZeoliteX. The matrix is preferably an acidic matrix. The acidic matrix willsuitably comprise amorphous alumina and preferably more than 10 wt % ofthe catalyst is amorphous alumina. The matrix may further comprise, forexample, aluminium phosphate, clay and silica and mixtures thereof.Amorphous alumina may also be used as a binder to provide the matrixwith enough binding function to properly bind the molecular sieve.Examples of suitable catalysts are commercially available catalysts usedin fluid catalytic cracking processes which catalysts comprise a ZeoliteY as the molecular sieve and at least alumina in the matrix.

The temperature at which feed and catalyst contact is between 450 and650° C. More preferably the temperature is above 475° C. and even morepreferably above 500° C. Good gasoline yields are seen at temperaturesabove 600° C. However higher temperatures than 600° C. will give rise tothermal cracking reactions and the formation of non-desirable gaseousproducts like for example methane and ethane. For this reason, thetemperature is more preferably below 600° C. The process may beperformed in various types of reactors. Because the coke make isrelatively small, as compared to an FCC process operating on apetroleum-derived feed, it is possible to conduct the process in a fixedbed reactor. In order to be able to regenerate the catalyst more simply,preference is nevertheless given to either a fluidised bed reactor or ariser reactor. If the process is performed in a riser reactor, thepreferred contact time is between 1 and 10 seconds and more preferredbetween 2 and 7 seconds. The catalyst to oil ratio is preferably between2 and 20 kg/kg. It has been found that good results may be obtained atlow catalyst to oil ratios of below 15 and even below 10 kg/kg.

This is advantageous because this means a higher productivity percatalyst resulting in, e.g. smaller equipment, less catalyst inventory,less energy requirement and/or higher productivity.

The catalyst system may advantageously also comprise of a medium poresize molecular sieve such to also obtain a high yield of propylene andother lower olefins next to the gasoline fraction. It has also beenfound that the yield to gas oil increases when such medium poremolecular sieves are present. Preferred medium pore size molecularsieves are zeolite beta, Erionite, Ferrierite, ZSM-5, ZSM-11, ZSM-12,ZSM-22, ZSM-23 or ZSM-57. The weight fraction of medium pore crystals onthe total of molecular sieves present in this process is preferablybetween 2 and 20 wt %. The medium pore molecular sieve and the largepore molecular sieve may be combined in one catalyst particle or bepresent in different catalyst particles. Preferably, the large andmedium pore molecular sieves are present in different catalyst particlesfor practical reasons. For example, the operator can thus add the twocatalyst components of the catalyst system at different addition ratesto the process. This could be required because of different deactivationrates of the two catalysts. A suitable matrix is alumina. The molecularsieve may be dealuminated by for example steaming or other knowntechniques.

It has been found that the combination of the large pore molecularsieve, more preferably of the FAU type, in combination with the mediumpore size molecular sieve, results in a high selectivity to the lowerolefins. Applicants have found that, by performing the process accordingthe invention with a large pore molecular sieve, more preferably of theFAU type, in combination with the medium pore size molecular sieve, asdescribed above, not only lower olefin yield improves, but also theyield to the iso and normal pentenes and hexenes increases. In such anembodiment these pentenes and hexenes are preferably oligomerised tocompounds boiling in the gas oil range. This is preferred for at leasttwo reasons, namely that the ultimate yield to gas oil increases andalso because low octane contributing compounds are removed from thegasoline. Oligomerisation is a well known process and is for exampleexemplified in US-A-20020111521.

In step (c) a second gas oil fraction is isolated from the product ofstep (b) from the main gasoline product. Isolation of said fractions issuitably performed by means of distillation. In this invention agasoline or gasoline fraction is a fraction boiling for more than 90 wt% between 25 and 215° C., preferably boiling for more than 95 wt % insaid boiling range. A gas oil or gas oil fraction is a fraction boilingfor more than 90 wt % between 200 and 370° C., preferably boiling formore than 90 wt % between 215 and 350° C.

The first and second gas oil fraction may separately or in a mixture besubjected to an additional catalytic dewaxing step in order to reducethe pour point to an acceptable level if required. Such a treatment isnot only advantageous for reducing the pour point but will also decreasethe content of any aromatic compounds formed in step (a). The pour pointis preferably below −10° C. and even more preferably below −15° C.Catalytic gas oil dewaxing may suitably be performed using a catalystcomprising a binder, a molecular sieve and a hydrogenation metalcomponent. The binder may be any binder, suitably alumina,silica-alumina or silica. The molecular sieve is preferably a zeolite ora silica-aluminophosphate (SAPO) material. The zeolites preferably havea pore diameter of between 0.35 and 0.8 nm. Suitable intermediate poresize zeolites are mordenite, Zeolite Beta, ZSM-5, ZSM-12, ZSM-22,ZSM-23, MCM-68, SSZ-32, ZSM-35 and ZSM-48. Preferredsilica-aluminophosphate (SAPO) materials are SAPO-11. The hydrogenationcomponent is preferably a Group VIII metal, more preferably nickel,cobalt, platinum or palladium. Most preferably the noble metal GroupVIII metals are used. Catalytic dewaxing conditions are known in the artand typically involve operating temperatures in the range of from 200 to500° C., suitably from 250 to 400° C., hydrogen partial pressures in therange of from 10 to 200 bar, preferably from 15 to 100 bar, weighthourly space velocities (WHSV) in the range of from 0.1 to 10 kg of oilper litre of catalyst per hour (kg/l/hr), suitably from 0.2 to 5kg/l/hr, more suitably from 0.5 to 3 kg/l/hr and hydrogen to oil ratiosin the range of from 100 to 2,000 litres of hydrogen per litre of oil.Examples of suitable dewaxing processes and catalysts are described inWO-A-200029511 and EP-B-832171.

EXAMPLES A-D

A Fischer-Tropsch product having the properties as listed in Table 1 wascontacted with a hot regenerated catalyst at different temperatures andcontact times at a catalyst to oil ratio of 4 kg/kg. The catalyst was acommercial FCC catalyst comprising an alumina matrix and Ultra StableZeolite Y, which had been obtained from a commercially operating FCCunit. The Zeolite Y content was 10 wt %. The operating conditions arepresented in Table 3. TABLE 1 Initial boiling point 100° C. Fractionboiling between 25 and 215° C. (wt %) 46.8 Fraction boiling between 215and 325° C. (wt %) 42.2 Fraction boiling above 325° C. (wt %) 11.0

EXAMPLES 1-4

A Fischer-Tropsch product having the properties as listed in Table 2 wascontacted with a hot regenerated catalyst at different temperatures andcontact times as in Examples A-D. The Fischer-Tropsch product wasobtained according to Example VII using the catalyst of Example III ofWO-A-9934917. The operating conditions are presented in Table 3. TABLE 2Initial boiling point 280° C. Weight Fraction having 10 or less carbon 0atoms(%) Weight Fraction having more than 30 carbon 80 atoms(%) WeightFraction having more than 60 carbon 50 atoms(%) Ratio of C₆₀+/C₃₀+ 0.63

TABLE 3 Temperature Contact Time Experiment Example (° C.) (seconds) A 1500 4.06 B 2, 5 525 0.7 C 3, 6 525 4.06 D 4, 7 625 0.7

TABLE 4 Middle Gasoline distillate Gasoline normal yield (wt % on yield(wt % on and iso-pentenes total total (wt % in gasoline Example product)(*) product) (**) fraction) A — — — 1 74.00 11.06 16.92 B 52.58 35.38 2.01 2 52.90 13.27 18.85 C 68.70 13.63 13.66 3 70.29  5.91 39.75 D53.86 26.24 24.09 4 46.12  7.43 36.32(*) Gasoline fraction defined as the distillation cut boiling between 25and 215° C.(**) Middle distillate defined as the distillation cut boiling between215 and 325° C.

From Table 4, it can be derived that the process according to theinvention will provide high yields to gasoline and middle distillate, orgas oil. In Examples 1-4, gas oil yields are lower than in Examples B-D,but the gas oil content in the feed to experiments B-D is 42.2 wt %(Table 1), which is higher than the gas oil yield in any of experimentsB-D. In addition, the gasoline fractions from experiments 1-4 containconsiderable amounts of normal and iso-pentenes, which can beoligomerised to gas oil.

Table 4 also shows that a high gasoline yield is obtained at highcontact times and relatively mild temperatures (Examples B and 2).

EXAMPLES 5-7

Examples 2-4 were repeated with the Fischer-Tropsch product having theproperties as listed in Table 5 and the conditions of Table 3. The feedin Table 5 can be obtained from the feed in Table 2, by removing 22 wt %of the gas oil and lighter fraction of Table 1. The yields are presentedin Table 6. The gas oil yields are higher than the yields in Examples2-4, but considerably lower than the sum of the gas oil yields fromExamples 2-4 and the 9 wt % (on total feed) gas oil that can berecovered from the fraction of Table 1, and blended with the gas oilfractions obtained in Examples 2-4, according to the invention. TABLE 5Initial boiling point 100° C. Weight Fraction having 10 or less carbon14 atoms (%) Weight Fraction having more than 30 carbon 62 atoms(%)Weight Fraction having more than 60 carbon 39 atoms(%) Ratio ofC₆₀+/C₃₀+ 0.63

TABLE 6 Middle Gasoline distillate Gasoline normal yield (wt % yield (wt% on and iso-pentenes on total total (wt % in gasoline Example product)(*) product) (**) fraction) 5 52.85 16.57 16.25 6 70.05 7.04 35.73 747.25 10.18 34.40(*) Gasoline fraction defined as the distillation cut boiling between 25and 215° C.(**) Middle distillate defined as the distillation cut boiling between215 and 325° C.

EXAMPLE 8

Example 6 was repeated except that part of the catalyst was exchangedfor a 25 wt % ZSM-5 containing catalyst. The content of ZSM-5 basedcatalyst on the whole catalyst charge was 20 wt % (as calculated on thetotal catalyst weight). The gasoline yield was 47.99 wt %, and themiddle distillate yield 9.27 wt % on total product. The content ofnormal and iso-pentenes was 54.61 wt % in the gasoline fraction.

EXAMPLE 9

Example 2 was repeated except that part of the catalyst was exchangedfor a 25 wt % ZSM-5 containing catalyst. The content of ZSM-5 basedcatalyst on the whole catalyst charge was 20 wt % (as calculated on thetotal catalyst weight). The results are presented in Table 7.

EXAMPLE 10

Example 3 was repeated except that part of the catalyst was exchangedfor a 25 wt % ZSM-5 containing catalyst. The content of ZSM-5 basedcatalyst on the whole catalyst charge was 20 wt % (as calculated on thetotal catalyst weight). The results are presented in Table 7. TABLE 7Middle Gasoline distillate Gasoline normal yield (wt % yield (wt % onand iso-pentenes on total total (wt % in gasoline Example product) (*)product) (**) fraction) 2 52.90 13.27 18.85 3 70.29 5.91 39.75 9 55.8813.39 11.47 10 45.76 8.07 67.14(*) Gasoline fraction defined as the distillation cut boiling between 25and 215° C.(**) Middle distillate defined as the distillation cut boiling between215 and 325° C.Example 8-10 show that the addition of ZSM-5 increases oil yields.

1. A process to prepare a gas oil, by (a) isolating from aFischer-Tropsch synthesis product a first gas oil fraction and a heavierfraction boiling above the gas oil fraction; (b) contacting the heavierfraction with a catalyst system comprising a catalyst, which catalystcomprises an acidic matrix and a large pore molecular sieve in a riserreactor at a temperature of between 450 and 650° C. at a contact time ofbetween 1 and 10 seconds and at a catalyst to oil ratio of between 2 and20 kg/kg; (c) isolating from the product of step (b) a second gas oilfraction; and (d) combining the first gas oil fraction with the secondgas oil fraction.
 2. The process according to claim 1, wherein theheavier fraction used in step (b) has a weight ratio of compounds havingat least 60 or more carbon atoms, and compounds having at least 30carbon atoms, of at least 0.2, and wherein at least 30 wt % of thecompounds have at least 30 carbon atoms.
 3. The process according toclaim 2, wherein at least 50 wt % of the compounds in the heavierfraction used in step (b) have at least 30 carbon atoms.
 4. The processaccording to claim 3, wherein the weight ratio of compounds having atleast 60 or more carbon atoms, and compounds having at least 30 carbonatoms, in the Fischer-Tropsch product is at least 0.4, in the heavierfraction used in step (b).
 5. The process according to claim 1, whereinthe temperature in step (b) is below 600° C.
 6. The process according toclaim 1, wherein the acidic matrix is alumina.
 7. The process accordingto claim 1, wherein the large pore molecular sieve is of the Faujasitetype.
 8. The process according to claim 1, wherein the catalyst systemin step (b) also comprises zeolite beta, Erionite, Ferrierite, ZSM-5,ZSM-11, ZSM-12, ZSM-22, ZSM-23, or ZSM-57.
 9. The process according toclaim 8, wherein iso and normal pentenes and/or iso and normal hexenesproduced in step (b) are subjected to an oligomerisation step to preparecompounds boiling in the gas oil range and wherein said compounds arecombined with the gas oil product as obtained in step (d).
 10. Theprocess according to claim 1, wherein the Fischer-Tropsch synthesisproduct used as feed in step (a) is obtained by means of acobalt-catalyzed Fischer-Tropsch synthesis process.
 11. The processaccording to claim 10, wherein the cobalt catalyst is obtained by (aa)mixing (1) titania or a titania precursor, (2) a liquid, and (3) acobalt compound, which is at least partially insoluble in the amount ofliquid used, to form a mixture; (bb) shaping and drying of the mixturethus obtained; and (cc) calcination of the composition thus obtained.